Southern and black, in history and in story; A Gathering of Old Men, by Ernest J. Gaines. New York:Alfred A. Knopf. 214 pp. $ 13.95. Lemon Swamp and Other Places: A Carolina Memoir, by Mamie Garvin Fields with Karen Fields. New York: Free Press. 250 pp. $15.95.

Two new books by black Americans - one fiction, the other an oral memoir - look at the Southern past with anguish and pride. In Gaines's novel, black men want to do something worthwhile with their lives; Ms. Fields's memoir is concerned with ''how we led our lives, how we led good lives.'' Together, the books provide an unusual and challenging portrait of a changing South. In both books, the characters recall the periods before and after World War II. But the points of view are as different as they are moving although both are, surprisingly, positive.

In ''A Gathering of Old Men,'' a white woman and 17 black men all plead guilty to the murder of a Cajun farmer. Neither threats from the white sheriff nor the promise of violence from vigilante groups can persuade the men to leave the scene of the crime or destroy the memories of indignity and abuse which bring them together.

Gaines writes in deft and accurate language, conveying the personality of a town and its people. His characters - both black and white - understand that, before the close of the novel, the new South must confront the old, and all will be irrevocably changed. Gaines portrays a society that will be altered by the deaths of its ''old men,'' and so presents an allegory about the passing of the old and birth of the new.

''A Gathering of Old Men'' traces black literature from Richard Wright's ''Native Son,'' a youthful angry fiction of masculinity, through Gaines's own ''Autobiography of Miss Jane Pittman,'' as the last defiant act of an elderly woman, to the acceptance of self by ''old men'' who will not be regarded as ''boys'' any longer. Gaines's novel recalls the stubborn dignity of Faulkner's hero in ''Intruder in the Dust,'' but has gone one step further in having depicted stubborn men who create, in their twilight years, a sense of community.

''Lemon Swamp and Other Places'' is a book of memories, a series of pictures, and an oral history. The memory is vivid and clear as the life of a woman, Mamie Garvin Fields, and her family. With the assistance of her granddaugter Karen Fields, she presents the pride and accomplishments of a Southern family, its relatives and neighbors, living through slavery, world wars, and Jim Crow laws. The book is a reminder to us all that history is filled with forgotten people whose memories so often die with them.

''Lemon Swamp'' began as a packet of papers entitled ''Letters to My Three Granddaughters,'' which shared the family's history in a manner that Alex Haley's ''Roots'' could never achieve. This is not a heroic quest for a lost country, a purpose, and sense of self: The Fields family knows who and what they are. Ms. Fields writes about her life as something outside history textbooks. She shares her memories of a South where blacks educated one another during slavery, long before the abolitionists came South to ''help the Negro.''

This is the history of a family who worked as educators and never allowed the white supremacists to denigrate them summarily, for they had their own method of handling prejudice with dignity.

Fields presents images of the black soldier during World War I, of teaching school when books were segregated, and of communities where it was possible to leave one's doors unlocked and feel safe. She speaks sadly of the decline of community and family replaced by social service agencies; of how good manners may conceal fear; of color prejudice and sexism among blacks. There is also a moving and heroic portrait of Mary Macleod Bethune, a black leader who reminded all people of their rights, of the promise of education, and of the importance of the vote.

Both the Gaines and Fields books are important additions to the literature of black Americans. Ernest Gaines has given us his most mature fiction to date, and Mamie Garvin Fields reminds us of the important contributions to history made by ''ordinary Americans.''